The level of a liquid within a tube or against a window has been used for many years to provide a visual indication of a process variable, etc. Some examples of the foregoing are a thermometer for measuring temperature, a manometer for measuring pressure and sight glasses for determining liquid levels within a container. The latter are used in steam boiler installations to provide a visual indication of the water level within the boiler. The foregoing devices are adequate if the operator is near the boiler, however, control rooms for such boiler installations are now being located remotely from the boiler. Thus, systems had to be developed for the transmission of such water level measurements to the remote control rooms. These systems typically utilize lenses, beam-splitters, etc. to transmit light, which passes through or is reflected from these measuring devices, to a viewing screen within the control room. As such, these systems are complex and relatively expensive because of the apparatus required to detect and transmit the light from the measuring device to the control room.
Because of the foregoing, it has become desirable to develop a system which utilizes dependable, inexpensive apparatus for the transmission of light from the measuring device to the viewing screen and which will also provide for the actuation of an alarm(s) if the process variable being monitored varies from a predetermined range.